1. Field of the Invention
The present invention relates to a spindle motor suitable for use in a rotary drive unit, wherein the rotary drive unit is used in office automation equipment such as computers and the like, and in their peripheral equipment as well.
2. Description of the Related Art
As shown in FIG. 4, a conventional spindle motor, which serves as a magnetic disk drive component in a hard disk drive unit (one of computer peripheral equipments), has a construction in which: an upright shaft 42 has its base portion fixedly mounted in a base 41; and, a rotor hub 43 forming a rotating member of the motor is rotatably mounted on the upright shaft 42 through a pair of an upper ball bearing 44 and a lower ball bearing 45, inner rings 44a, 45a of which bearings 44, 45 are axially spaced apart from each other, wherein the inner rings 44a and 45a belong to the upper ball bearing 44 and the lower ball bearing 45, respectively, and have conventional steel balls 48a and 48b interposed between the inner rings 44a, 45a and their corresponding outer rings 44b, 45b, respectively; these ball bearing 44 and 45 have their outer rings 44b and 45b fitted in a sleeve 46, respectively; the sleeve 46 is fitted in an axial center bore 47 of the rotor hub 43; and, the ball bearings 44, 45 have their inner rings 44a, 45a or their outer rings 44b, 45b bonded to the upright shaft 42 or to the sleeve 46 using an appropriate adhesive or like fastening means in a condition in which the inner ring 44a or the outer ring 44b of the upper ball bearing 44 is appropriately pressed from above in a manner such that the balls 48a, 48b are brought into press-contact with both the inner and the outer rings of these ball bearings 44, 45.
When the spindle motor is energized, a temperature of the motor goes up so that its individual components thermally expand at various expansion amounts. The relationship in thermal expansion amount in radial direction of these components of the ball bearings 44, 45 is as follows: outer rings greater than inner rings greater than balls.
Consequently, when the temperature of the spindle motor goes up, a radial space or clearance between the inner ring and the corresponding outer ring in each of the ball bearings 44, 45 increases. On the other hand, since the balls 48a, 48b are smaller in thermal expansion amount than the inner and the outer rings, a pre-load applied to each of the balls 48a, 48b by the inner and the outer rings is reduced, which results in variations in resonant frequency of the motor, and often results in the generation of the resonant vibration of the motor with a structural member to which the motor is mounted.
For example, in case that the conventional motor is used as a drive component of the hard disk drive unit, there is a fear that the conventional motor resonates to the structural member such as a swing arm, a casing and the like to impair the hard disk drive unit in accuracy in data reading and writing operations due to the drive unit""s vibration and also to impair the hard disk drive unit in quietness in operation due to noises caused by the drive unit""s vibration.
On the other hand, when differences in thermal expansion amount between the inner rings and the outer rings of the ball bearings 44, 45 further increase, too much play appears between the balls and each of the inner and the outer rings, which causes considerable vibrations in rotation of the rotor hub 43. Such vibrations of the rotor hub 43 cause a recording surface of the magnetic disk to vibrate in axial direction and considerably impairing the hard disk drive unit in reliability in operation.
Particularly, when the balls 48a, 48b are made of ceramics in order to improve the hard disk drive unit in reliability, the above-mentioned problem caused by the differences in thermal expansion amount among the individual components of the ball bearings becomes more serious, because the ceramic balls are much less in thermal expansion amount than the conventional steel balls.
It is an object of the present invention to provide a spindle motor, which is capable of keeping high rotation accuracies and having its inner and its outer rings brought into appropriate press-contact with its balls even when a temperature of the motor goes up to permit individual components of the motor to thermally expand at various expansion amounts, and further capable of being free from variations in resonant frequency and also free from vibrations in rotation.
In accordance with a first aspect of the present invention, the above object of the present invention is accomplished by providing:
In a spindle motor comprising: a first upright shaft fixedly mounted on abase; a pair of a first upper ball bearing and a first lower ball bearing both rotatably mounted on the first upright shaft and provided with a plurality of first balls between their first inner rings and their first outer rings; a first sleeve fitted to outer peripheral surfaces of the first outer rings; and, a rotor hub mounted on an outer peripheral surface of the first sleeve so that the rotor hub is rotatably supported on the base, the improvement wherein:
the rotor hub is provided with an axial center bore;
the axial center bore is provided with a pair of large-diameter bore portions in its opposite axial end portions together with a small-diameter bore portion which is axially interposed between the large-diameter bore portions;
an inner diameter of the small-diameter bore portion is smaller than an outer diameter of the first sleeve;
the axial center bore of the rotor hub is fitted to an outer peripheral surface of the first sleeve in an insertion manner through a heating shrinkage fit process, so that the first sleeve is subjected to a radially inwardly exerted pressure applied from outside to the first sleeve to deform resiliently radially inwardly;
whereby a small-diameter portion is formed in the first sleeve to extend radially inwardly between the first outer rings of the first upper ball bearing and the first lower ball bearing.
In the spindle motor of the present invention having the above construction, preferably, an annular groove is formed in an inner peripheral surface of the first sleeve in an axial position in which each of the first outer rings of the ball bearings is brought into contact with the inner peripheral surface of the first sleeve; and, the annular groove is substantially equal in axial width to an outer ring raceway of each of the first outer rings, wherein the outer ring raceway is formed in an inner peripheral surface of each of the outer rings.
In accordance with a second aspect of the present invention, the above object of the present invention is accomplished by providing:
In a spindle motor comprising: a stepped shaft i.e., a two-diameter shaft, provided with a large-diameter portion and a small-diameter portion; a second sleeve which surrounds the stepped shaft and provided with a pair of an upper outer ring raceway and a lower outer ring raceway both arranged parallel to each other in an inner peripheral surface of the second sleeve; an upper row of balls interposed between an upper inner ring raceway and the upper outer ring raceway of the second sleeve, wherein the upper inner ring raceway is formed in an outer peripheral surface of an inner ring which is fitted to the small-diameter portion of the stepped shaft in an insertion manner; a lower row of balls interposed between a lower inner ring raceway and the lower outer ring raceway which is formed in a lower portion of the inner peripheral surface of the second sleeve, wherein the lower inner ring raceway is formed directly in an outer peripheral surface of the large-diameter portion of the stepped shaft; the second sleeve, the balls, the inner ring and the stepped shaft are combined with each other to form a compound bearing assembly, wherein a base portion of the large-diameter portion of the stepped shaft in the compound bearing assembly is uprightly and fixedly mounted in a base; and, a rotor hub mounted on an outer peripheral surface of the second sleeve so that the rotor hub is rotatably supported on the base, the improvement wherein:
the rotor hub is provided with an axial center bore;
the axial center bore is provided with a pair of large-diameter bore portions in its opposite axial end portions together with a small-diameter bore portion which is axially interposed between the large-diameter bore portions;
an inner diameter of the small-diameter bore portion is smaller than an outer diameter of the second sleeve;
the axial center bore of the rotor hub is fitted to an outer peripheral surface of the second sleeve in an insertion manner through a heating shrinkage fit process, so that the second sleeve is subjected to a radially inwardly exerted pressure applied from outside to the second sleeve to deform resiliently radially inwardly;
whereby a small-diameter portion is formed in the second sleeve to extend radially inwardly between the upper outer ring raceway and the lower outer ring raceway of the second sleeve.
In the spindle motor of the present invention having the above construction, preferably: the inner ring is equal in an outer diameter to the large-diameter portion of the stepped shaft; and, the balls of the upper row thereof are equal in a ball diameter to the balls of the lower row thereof.
Further, preferably, the balls are made of ceramics.
The spindle motor of the present invention has the following effects: namely
In the spindle motor of the present invention, the small-diameter portion is formed in an axially intermediate portion of each of the first and the second sleeve, for example: by radially inwardly pressing the first sleeve""s intermediate portion between the opposite axial end portions of the first sleeve which surrounds the upper and the lower outer ring; or, by radially inwardly pressing the second sleeve""s axially intermediate portion between the upper and the lower outer ring raceway of the second sleeve which serves as a common outer ring of the compound ball bearing assembly. Consequently, when a temperature of the spindle motor goes up due to its energization, the individual components of the spindle motor thermally expand at various expansion amounts to increase a space or clearance between the inner ring raceway and the outer ring raceway in each of the upper and the lower ball bearings or in each of the upper and the lower ball bearing portions of the compound ball bearing assembly. However, even when the clearance between the inner ring raceway and the outer ring raceway increases, it is possible for the spindle motor of the present invention to keep its ball""s pre-load constant, because each of the first and the second sleeve is capable of being resiliently deformed due to its resiliency, and therefore capable of keeping constant a pressure applied from each of the inner and the outer ring raceway to the balls.
Consequently, it is possible for the spindle motor of the present invention to keep its accuracy in rotation at high levels in a steady manner even when the spindle motor has its temperature increased in operation. Further, it is also possible for the spindle motor of the present invention to substantially prevent its resonant frequency from varying, which prevents the motor from vibrating in rotation, and therefore prevents noises from being generated in the motor in operation due to the vibrations created by the motor.
Further, in the second sleeve provided with the outer ring raceways in its inner peripheral surface, it is possible for the balls of the compound ball bearing assembly to keep their rotation in a steady manner, because the provision of the outer ring raceways in the second sleeve may spread the ball""s load more evenly even when the balls are subjected to excessive loads applied by the outer ring raceways of the second sleeve due to thermal deformations of the second sleeve and the rotor hub.
Still further, when the balls are made of ceramics it is possible to remarkably improve such ceramic balls in durability in comparison with the ordinary steel balls, which leads to a remarkable improvement of the spindle motor of the present invention in service life.